Gas filled rubber play balls for tennis and other games are well known. They contain gases at atmospheric pressure or at elevated pressures. Tennis balls filled with compressed gases are preferred by many players due to their perceived playing properties. Typically, the balls start out with gases at pressures of 10 psi (0.7 kg f/cm.sup.2, 0:069 MPa) above atmospheric pressure and then lose pressure during storage and use. The loss of pressure causes the playing characteristics of the ball to change.
An alternative to pressurized balls are balls with an internal pressure near atmospheric pressure. These are known as pressureless balls. To get playing characteristics similar to a pressurized ball, pressureless balls tend to have thicker walls than pressurized balls. To maintain the same ball weight with thicker walls, the density of the rubber compound in the wall needs to be lower.
Prior art patents substituted thermoplastic polymeric fillers, such as polystyrene, for mineral type fillers due to the lower density of thermoplastics than the many mineral type fillers. The high melting temperatures of these thermoplastics require an extra processing step where the thermoplastics were blended with the elastomer and dispersed at the higher temperature where the thermoplastic softens.
A problem with many rubber ball compounds is the tendency of the ball to soften (the modulus of the cured ball decreases) as a result of the repeated deformation during play. This changes the play characteristics of the ball. A reduced tendency to soften is clearly an advantage.